This invention relates to an electromagnet and more particularly to an electromagnet with a magnetic shield for generating a highly homogeneous high magnetic field for use in a nuclear magnetic resonance diagnosis system.
In an electromagnet for a nuclear magnetic resonance diagnosis system, a uniform magnetic field of a high intensity of the order of from 0.5 Tesla to 2 Tesla is necessary in an imaging space within an opening of the electromagnet. On the other hand, upon generating such intense magnetic field, a problem that the magnetic field leaks to the exterior of the electromagnet and gives undesirable affects to the surrounding equipment occurs. Therefore, it is customary to enclose the electromagnet with a ferromagnetic member called a magnetic yoke to magnetically shield the electromagnet.
FIGS. 1 and 2 are a sectional view and a sectional side view, respectively, of a conventional electromagnet for a nuclear magnetic resonance diagnosis system put in practical use from the point of view as above discussed.
In these figures, the conventional electromagnet comprises a hollow cylindrical superconducting magnetic field coil 1, a ring-shaped cryogenic vessel 2 for maintaining the magnetic field coil 1 at a cryogenic temperature. The electromagnet also comprises a magnetic shield 3 which is composed of a pair of ring-shaped magnetic end plates 4 with a circular central opening 5 formed therein and a cylindrical yoke 6 of a magnetic material magnetically and mechanically connected at its both end to the end plates 4 so that a magnetic circuit is defined around the magnetic field coil 1.
The magnetic shield 3 must have a cross-sectional area sufficient for accommodating the magnetic flux generated by the magnetic field coil in order to concentrate the magnetic flux to the central region of the electromagnet and to prevent leakage of the magnetic flux. Therefore, the cylindrical magnetic yoke 6 must have a wall thickness of a certain mean value, for a given electromagnet, necessary for providing a neccessary cross-sectional area for effectively accommodating the magnetic flux generated by the magnetic field coil.
Since the conventional electromagnet is constructed as above described, the leakage of the magnetic flux from the nuclear magnetic resonance diagnosis system can be significantly reduced. On the other hand, since the electromagnet for the nuclear magnetic resonance diagnosis system requires to have a highly uniform static magnetic field of the order of 10 ppm within a center sphere of 35 cm in the system, the magnetic shield 3 as well as the magnetic field coil 1 must be very precisely configured and positioned. The magnetic field coil 1 and the magnetic shield 3 generate a magnetic gradient which deteriorates the uniformity of the magnetic field when they are asymmetric with respect to the coil center and the coil axis, so that the magnetic field coil 1 and the magnetic shield 3 are coaxially disposed and arranged in symmetry also with respect to the longitudinal central axis of the magnetic field coil 1.
On the other hand, in an example shown in FIGS. 3 and 4 which is the one disclosed in Japanese Paten Laid-Open No. 60-90546, the electromagnet has a magnetic shield 7 which comprises four rectangular magnetic yokes 8 arranged at equal intervals between polygonal end plates 9 for the purpose of providing connecting side openings for the evacuating pump for the cryogenic vessel through the spaces between the magnetic yokes 8. This arrangement can be considered as an example in which there is no magnetic yoke positioned in the widthwise direction of the electromagnet.
With such the arrangement, magnetic reluctance of the magnetic path for the magnetic flux greatly varies from position to position because of the discontinuity of the magnetic yokes in the circumferential direction, so that, in order to realize a highly uniform magnetic field in the imaging space within the electromagnet for the nuclear magnetic resonance diagnosis system, it is necessary to increase the correction value of the separately provided magnetic field correction coil. This requires an increased power and additional provision of a correction coil. Also, the magnetic shielding effect of the magnetic shield 7 is relatively small.
While the conventional electromagnet is constructed as above described, as a stronger magnetic field of a higher uniformity is required, an increased ferromagnetic plate thickness for the magnetic shield is required, and the overall dimensions of the electromagnet are becoming as large as 2.5 m (width) .times.2.5 m (height) .times.2.5 m (length). In particular, in the case of the nuclear magnetic resonance diagnosis system, the system is often installed in a hospital or the like, where the space in which the system is installed is not necessarily large because of various reasons.
Also, when this system is to be additionally installed in an already-built building, problems arise such that the passage for carrying the electromagnet into the diagnosis room is too narrow in width as compared to height or the diagnosis room has too small floor area as compared to ceiling height, so that the carrying and installing are very difficult.